Low noise amplifiers (LNAs) are used in communication systems to amplify received signals, which tend to be very weak in signal strength. As the name suggests, one of the most important performance factors for LNAs is the amount of noise introduced into the amplified signals by the LNAs.
A typical LNA with an n-channel output transistor includes a bias circuit that provides a bias signal for the n-channel output transistor through a bias signal path. The amount of noise introduced by the LNA is quantified by the Noise Figure. The Noise Figure (NF) for a LNA is mathematically expressed as:
      NF    =          10      ⁢                          ⁢              log        ⁡                  (                                    S              i                                      S              O                                )                      ,where Si is the input signal-to-noise ratio and So is the output signal-to-noise ratio. Thus, external noise picked up at the input of the LNA will degrade Si, and consequently, degrade the NF of the LNA. It is therefore important to minimize any noise contribution at the input, which is primarily due to the bias circuit.
One technique to minimize the noise contribution at the input of the LNA from the bias circuit is to place a capacitor between the bias signal path and ground to shunt some of the noise on the bias signal path to ground. The effectiveness of this shunt capacitor depends on its size. However, due to limited real estate on the integrated circuit on which the LNA is fabricated, placing an effective shunt capacitor in the LNA is not practical.
Another technique is to place a resistor on the bias signal path between the source and the n-channel output transistor, which helps to isolate the noise of the bias circuit from the input of the LNA. However, a large resistor on the bias signal path will degrade large signal performance of the LNA, for example, reducing saturated output power. In addition, a large resistor on the bias signal path will also contribute noise at the input the LNA since noise caused by a resistor is proportional to its resistance. An inductor could be used in place of the resistor, but the inductance needed at lower frequencies, such as 2 GHz, would require a larger-than-practical inductor. Furthermore, such an inductor would significantly increase the series resistance, and thus, would introduce noise at the input of the LNA.
In view of these concerns, there is a need for a LNA and method for amplifying an input signal that reduces the noise introduced by the bias circuit of the LNA without affecting the signal performance of the LNA.